In recent years, fiber to the home (FTTh) and fiber to the curb (FTTx) gradually become the focuses in the construction of optical fiber networks. Thorough researches have been made on various optical fibers that might be used in the FTTx field. Currently, single-mode optical fibers have found wide application in networks, and with the wide application of low water peak single-mode optical fibers, low water peak optical fibers having bend insensitive performance gradually attract people's attention. An existing conventional low water peak optical fiber (meeting ITU-T G.652C/D) generally has a bend radius of 30 mm, resulting in severe limitations on cabling indoors and in narrow environments. Compared with a long distance transmission application, optical fibers indoors and in narrow environments are subject to high bending stresses. Especially, in use, optical fibers are often wound in storage boxes that become increasingly small, resulting in even higher bending stresses. Therefore, it becomes necessary to design and develop an optical fiber having high bending resistance performance to meet the requirements for FTTx network cabling and device miniaturization. In November 2009 and June 2010, ITU-T has amended the bend insensitive G.657 optical fiber standard twice and added a research report on lifetime performance of optical fibers having small bend radiuses (‘Characteristics of a bending loss insensitive single-mode optical fiber and cable for the access network’ and Amendment 1: Revised Appendix 1-Lifetime expectation in case of small radius bending of single-mode fiber). The two times of amendments have basically specified different application targets of the G.657A1/A2 optical fiber and the G.657.B3 optical fiber in different bend radius use environments. The G.657.A1 optical fiber that meets the minimum bend radius of 10 mm is applicable to long-haul networks. The G.657.A2 optical fiber meets applications on the condition of a minimum bend radius of 7.5 mm and is mainly applied in metro networks and FTTh. The G.657.B3 optical fiber meets the use condition of a minimum bend radius of 5 mm, is mainly applied in fiber to the desktop (FTTd) and all-optical networks and used in the manner of indoor optical fiber/optical cable, and focuses on the service life problem of optical fibers in a bending condition.
Technically speaking, the G.657 optical fiber is fully compatible with the G.652 optical fiber and has high macro-bending and micro-bending performance to completely replace G.652 optical fibers in wide use currently. Nowadays, the application of the G.657 optical fiber is mainly limited by high optical fiber cost, better bending performance, and the contradiction in its compatibility with the G.652 optical fiber. Therefore, on the condition of full downward compatibility with the G.652 optical fiber standard, to develop a G.657 optical fiber having higher bending performance and lower the production and fabrication cost of the G.657 optical fiber is of great meaning to the development of the G.657 optical fiber and the optical fiber access technology.
After years of researches, scientists and researchers all over the world have found that the mode field diameter and cut-off wavelength of an optical fiber play a major role in macro-bending loss of the optical fiber. A MAC value can qualitatively measure the bending performance of an optical fiber, in which MAC is defined as a ratio between a mode field diameter and a cut-off wavelength. When the MAC is smaller, the bending performance of the optical fiber is higher. Apparently, the object of lowering a MAC can be achieved by lowering a mode field diameter and increasing a cut-off wavelength of an optical fiber, so as to obtain high bending performance. U.S. publication No. 2007/007016, and Chinese patent Nos. CN1971321A, and CN1942793A adopt this type of methods. However, when a mode field diameter of an optical fiber is too small, a large connection loss occurs in its connection with a conventional single-mode optical fiber, and the incident optical power is limited. Also, in consideration of a multi-service characteristic of FTTx, it is expected to use the full band for transmission, and the cut-off wavelength of the optical cable has to be smaller than 1260 nm. Therefore, the space for the cut-off wavelength of the optical fiber to increase is very limited. Considering from the overall design of an optical fiber, it is one important direction for the research and development of the G.657 optical fiber to obtain a suitable MAC value on the basis of guaranteeing that the basic parameters of the optical fiber meet relevant ITU-T and IEC standards and the access performance of the optical fiber is stable by properly optimizing the sectional structure of the optical fiber, so as to achieve the highest bend insensitive performance for the optical fiber.
In contrast to the ordinary sectional structure of the single-mode optical fiber, another effective method of enhancing the bending performance of an optical fiber is to adopt a design of a depressed inner cladding layer. For example, the design of a depressed inner cladding layer is adopted in U.S. Pat. Nos. 5,032,001 and 7,043,125, and Chinese patent No. CN176680. Through the design of a depressed inner cladding layer, the numerical aperture (NA) of an optical fiber can be increased without increasing doping in the core layer, so as to avoid the increase of attenuation caused by increased doping. However, the optimized design of a depressed inner cladding layer can only improve the macro-bending performance of an optical fiber at a large bend radius to a certain extent. When the bend radius of an optical fiber is smaller than or equal to 10 mm, it is very difficult to adopt the method of a depressed inner cladding layer to prepare a bend insensitive optical fiber that meets the G.657.A2 standard. It is found through further researches that the most effective method of enhancing bending resistance performance of an optical fiber is to design the cross-section of an optical fiber by adopting a structure of a trench cladding layer, the basic waveguide structure thereof is described in U.S. Pat. Nos. 4,852,968, and 6,535,679 and Chinese patent No. CN1982928A also adopt the same type of design. However, all the above patents only consider how to lower a bending induced loss and none considers a long service life of the optical fiber at a small bend radius in combination with specific applications, and also none explicitly illustrates whether an optical fiber fabricated according to the specification thereof meets or goes beyond the relevant requirement of a minimum bend radius of 5 mm in the G.657.B3 standard. It is found through the research on an optical fiber having the structure of a trench cladding layer that certain requirements and limitations also exist about the depth and width of a trench cladding layer in the cross-section of an optical fiber: if the trench cladding layer is too shallow or too narrow, the desirable bend insensitive performance is not achieved; and if too deep or too wide, the cut-off wavelength and dispersion performance of an optical fiber might be affected. It should be noted that the latest researches indicate that: in an optical fiber link, especially an FTTx link, due to the existence of multiple bends and connectors, the phenomenon of a multi-path interference (MPI) might occur in the optical fiber. David Zhen et al. has introduced the method of testing an MPI in OFC/NFOEC (‘Testing MPI Threshold in Bend Insensitive Fiber Using Coherent Peak-To-Peak Power Method’) in 2009. Especially in the optical fiber design of an outer depressed cladding layer, if the depressed cladding layer is too close to the core layer, once a core layer offset occurs at an connector of an optical fiber, multi-path interferences occur easily. If the depressed cladding layer is too far away from the core layer, the effect of lowering the bending induced loss of the optical fiber cannot be achieved. Therefore, it is necessary to perform precise positioning on the depressed cladding layer. Hence, to properly design the cross-section of an optical fiber and obtain a desirable balance in the refractive index sectional structure of a core layer, a cladding layer, and a trench cladding layer is a focus and a challenge in the research of the G.657 optical fiber.
The fabrication cost of the G.657 optical fiber is mainly affected by prices of raw materials and fabrication efficiency of equipment. Nowadays, four methods are adopted to fabricate a typical preform of the G.657 optical fiber: modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD), outside vapor deposition (OVD), and vapor axial deposition (VAD). The MCVD and PCVD methods are inside deposition, and in depositing a trench cladding layer, being limited by the size of a bushing, it is usually very difficult to make the diameter of the preform to be greater than 100 mm. Chinese publication No. CN101585658A achieves a large-size perform by adding a small sleeve tube. Also, the inside deposition has a low rate, and the deposition thickness is too large, so that the efficiency of the equipment is obviously affected and the cost of the optical fiber is increased. In another aspect, Compared with outside deposition such as OVD and VAD, inside deposition processes such as PCVD and MCVD have the advantage of achieving deep fluorine doping, and also fluorine doping depth has high longitudinal and axial homogeneity. For the OVD and VAD processes that are outside deposition, in comparison, the advantages are a high deposition rate and a size being not limited by sleeve tube materials. However, if a fluorine-doped cladding layer needs to be fabricated in the process of depositing a core layer and an inner cladding layer, not only the process is difficult to control, but also, in the sintering process, because of the diffusion of fluorine, it is very difficult to perform effective control on the refractive index section. The method that is applicable in practical production is to deposit a core rod having a certain thickness of cladding layer first, perform dehydration and sintering, and deposit a fluorine-doped cladding layer on the glass core rod. Also, a deposition process may be adopted to directly perform fluorine doping, or perform fluorine doping in sintering. For example, in U.S. Pat. Nos. 5,895,515 and 4,579,571, the two methods are introduced, respectively. However, as both OVD and VAD are flame (H2/O2) hydrolysis methods, when being deposited on a glass core rod, a fluorine doping layer is inevitably directly exposed in a hydrogen/oxygen flame (H2/O2), a large number of hydroxyls generated on the H2/O2 flame diffuse to the core layer, which causes an increase to the water peak attenuation of the drawn optical fiber. Therefore, the cladding layer in the glass core rod needs to be thick enough to block the diffusion of hydroxyls to the inside. However, once the deposited cladding layer is too thick, the formed fluorine-doped cladding layer is too far away from the core layer to exert the effect of enhancing the bending performance of the drawn optical fiber. Also, it is very difficult to achieve deep fluorine doping in OVD and VAD processes, and also the longitudinal and axial homogeneity of the fluorine doping depth are low. In the four methods of fabricating a preform rod of an optical fiber, the requirement for the deposition of the core layer part is the strictest, and the core layer refractive index section and the material homogeneity need to be controlled precisely. The deposition of the trench cladding layer part requires more fluorine doping than other parts, the process control is stricter and the cost is higher as compared with a normal inner cladding layer or outer cladding layer.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.